1. Field of the Invention
The invention relates to a method of acquiring signals in a global navigation satellite system (GNSS).
2. Description of the Prior Art
In a global navigation satellite system such as the Global Positioning System (GPS), each satellite of a constellation of satellites broadcasts a signal carrying time and date information that is very accurate because it is obtained from an atomic clock on board the satellite. This signal is referred to hereinafter as a navigation signal. A navigation signal is the result of modulating a carrier by a pseudorandom spreading code and where applicable by a navigation message. The spreading code is used to distinguish between different navigation signals.
The receiver of a navigation terminal must acquire the navigation signals from at least three satellites in order to determine its position in three dimensions in a global system of absolute coordinates.
If it does not know the universal time accurately, the receiver of a navigation terminal must acquire a fourth navigation signal.
In practice, complex computations using the information carried by the acquired signals enables the terminal to determine its position and therefore to address the location problem.
This is not the problem that the present invention seeks to solve, which is that of acquiring the signal transmitted by a satellite in view.
In a preliminary phase preceding the location process as such, a terminal acquires navigation signals broadcast by the satellites in order to be able to perform the computations of the location process using those signals. The invention relates to this preliminary phase, which is called the navigation signal acquisition phase and also involves computations.
The present invention relates to the acquisition of the GNSS signals transmitted by the satellites by a receiver terminal called a navigation terminal.
Acquisition of the GNSS signals entails time and frequency scanning to correlate the received signal with a replica of the required signal over a particular time period that is a function of the signal to noise (S/N) ratio. A low S/N ratio necessitates a longer correlation time, which induces a higher frequency step resolution, and thus a greater number of frequencies to be scanned for the same Doppler dynamic range (the Doppler phenomenon is linked to the movement of the satellite and/or the terminal).
However, the correlation time is still limited by the accuracy of the frequency reference of the terminal.
The carrier frequency and the phase of the spreading code relative to a frequency and time reference are referred to hereinafter as navigation signal tuning parameters.
The main problem for a navigation terminal in acquiring navigation signals is the number of processing operations to be carried out, which is in inverse proportion to the signal to noise ratio.
A first prior art solution to this problem increases the number of correlators operating in parallel in each terminal. The drawbacks of this solution are the increased logical complexity of the electronic circuits and the increased dissipation of the receiver.
A second prior art solution uses a base station to provide the terminal with assistance by communicating to the terminal the identities of the satellites that are visible and the tuning parameters for the navigation signals associated with those satellites.
The time/frequency reference of the base station must be broadcast to the terminals to enable them to use the assistance information. The base station and the terminals communicate to enable the transmission of assistance information and time/frequency synchronization information over a radio-frequency (RF) link provided by a cellular or other connection.
The drawback of the second solution is the sensitivity of the radio-frequency link to interference, in particular when transmitting the time/frequency reference. Also, a link of this kind necessitates a particularly wide bandwidth for transmitting accurate time-frequency synchronization (of the order of 100 MHz for time synchronization to within a few tens of nanoseconds).
Moreover, the base station providing the assistance must first acquire navigation signals from the satellites in view, even though it is as sensitive to interference as the terminals that it is assisting.
The acquisition of navigation signals when the signal to noise ratio is low and within an acceptable time period (of the order of a few tens of seconds) therefore remains a problem. The acquisition phase represents a large processing load that is in inverse proportion to the signal to noise ratio.
Thus no prior art solution achieves satisfactory navigation signal acquisition performance (in terms of acquisition time, probability of non-acquisition, probability of false acquisition, required computation power) if the signal to noise ratio is low.
The problem addressed by the present invention is that of improving the performance of the navigation signal acquisition phase regardless of the signal to noise ratio and without increasing the computation power of the navigation terminals.
The invention solves this problem by constructing a network of nearby terminals communicating via a radio-frequency link channel to collaborate for the purpose of effecting the computations implementing the acquisition phase.
The terminals are “nearby” if they are spaced by a few wavelengths of the modulation by the spreading code, defined as the “chip” length. For example, in the GPS the chip length for a modulation frequency of 1.023 MHz is 300 m.
Compared to existing methods, this solution achieves the required sensitivity without increasing the duration of the acquisition phase, as well as robustness to interference and fast acquisition.
This solution also provides local and autonomous assistance for tuning an incoming terminal entering the network to navigation signals to which the network has already been tuned.
Reciprocally, the network can inherit tuning parameters that have already been acquired by an incoming terminal entering the network.